An electrolytic process is a process in which electrical energy, which is a clean energy, is utilized and chemical reactions occurring on the electrode surfaces can be controlled. Furthermore, since an oxidation reaction and a reduction reaction proceed at different places, i.e., on the anode and the cathode, reaction products can be isolated by a simple operation. The process is a basic technique generally used for the electrolysis of sodium chloride, electroplating, metal collection, metal refining, etc. Recently, the process is coming to be utilized for wastewater treatment, e.g., a treatment for making an organic wastewater harmless.
On the other hand, oxidation reactions at the anode yield substances effective in water treatment, such as oxidizing agents, e.g., effective chlorine and ozone, and active species, e.g., OH radicals. Water containing these substances hence is in general use under the name of active water, functional water, ion water, sterilization water, etc. Recently, this water has come to be called exclusively as electrolytic water.
Although electrolytic processes are being practically used as shown above, there are cases where intended reactions do not sufficiently proceed depending on the electrode materials. In general, in electrolysis in an aqueous solution, the water undergoes electric discharge to evolve oxygen and hydrogen respectively at the anode and cathode. In the case where the intended reaction is one involving neither oxygen evolution nor hydrogen evolution, it is necessary to minimize the progress of the water discharge reactions because the discharge of water reduces the efficiency of the intended reaction.
There are two methods for minimizing the progress of the water discharge reactions. One is to select an electrode material which facilitates the progress of the intended reaction, i.e., an electrode catalyst having a low overvoltage in the intended reaction. The other is to use an electrode material which makes the water discharge reactions less apt to proceed, i.e., an electrode catalyst having a high oxygen overvoltage or hydrogen overvoltage, to thereby make the progress of the intended reaction relatively easy.
Examples of the former method include to use a DSA type electrode generally used as an anode for sodium chloride electrolysis. DSA type electrodes have a low chlorine overvoltage and, hence, use of this type of electrode enables the desired chlorine generation to occur preferentially to oxygen evolution. This method is effective because the cell voltage is minimal and the energy efficiency is high accordingly. However, this method is not applicable when the equilibrium potential in the intended reaction is nobler than that in the oxygen evolution reaction or is less noble than that in the hydrogen evolution reaction. For example, this method is not applicable to a treatment for decomposing persistent substances because the oxidation potential for the persistent substances is nobler than the equilibrium potential in oxygen evolution. For causing such reactions to proceed, the latter method can be employed.
Examples of the latter method include to use the cathode for sodium chloride electrolysis conducted by the mercury process and use an anode for electrolytic ozone generation. In the mercury-process sodium chloride electrolysis, mercury, which has a high hydrogen overvoltage, is used as the cathode to thereby realize discharge of sodium ions. In ozone generation, lead dioxide, which has a high oxygen overvoltage, is used as the anode to thereby realize both oxygen evolution and ozone generation.
Even when an intended reaction can be caused to proceed preferentially by selecting an electrode material as described above, this electrolytic process cannot be practically employed as long as the electrode material selected has insufficient corrosion resistance. In particular, the corrosion resistance of the anode material on which an oxidation reaction is to be conducted is one of major problems encountered in putting the electrolytic process into practical use.
The materials usable as the substrate of the anode are substantially limited to valve metals, e.g., titanium Ti, and alloys thereof, and the electrode catalysts also are limited to noble metals such as platinum (Pt) and iridium (Ir), oxides of such noble metals, lead oxide (PbO2), and tin oxide (SnO2). It is, however, known that even when such expensive materials are used, they are consumed depending on the density of the current applied and on the lapse of time and dissolve away in the solution. There is hence a desire for an electrode having better corrosion resistance.
Graphite and amorphous carbon materials also have been used as electrode materials. However, carbon electrodes are consumable electrodes and are considerably consumed especially in anodic polarization. Consequently, use of carbon electrodes is limited to applications where consumption is allowable or where there is no substitute material.
Diamond is excellent in thermal conductivity, optical transparency, and resistance to high temperatures and oxidation, and the electrical conductivity thereof can be regulated especially by doping. Conductive diamond to which conductivity has been imparted by doping is hence regarded as a promising material for use in semiconductor devices and energy conversion elements.
Recently, it was ascertained that conductive diamond has a wider potential window in aqueous solutions than the known electrode catalysts. Many reports have been made on basic and practical electrochemical properties of diamond electrodes employing conductive diamond. In particular, since diamond electrodes are the highest in oxygen overvoltage among the presently known electrode materials, many investigations have been made on the use of a diamond electrode as an anode so as to take advantage of the high oxygen overvoltage (see, for example, Journal of Electrochemical Society, Vol.141, 3382- (1994) and Electrochemistry, p. 521, Vol.72, No.7 (2004)).
Based on such knowledge, references 1 and 2 suggest that when diamond is used as an anode material, an organic wastewater can be decomposed. Reference 3 proposes a technique in which conductive diamond is used as an anode and a cathode to electrochemically treat organic substances. Reference 4 proposes a technique in which a conductive diamond electrode and a gas diffusion cathode for hydrogen peroxide generation are used as an anode and a cathode, respectively, to conduct water treatment. Furthermore, reference 5 discloses a technique in which a diamond electrode is used for synthesizing electrolytic water and ozone gas.
It can be expected from those investigations that in electrolytic processes using diamond electrodes having a wider potential window than electrodes heretofore in use, the intended reactions proceed at a higher efficiency than in the case of using the existing electrodes. However, these electrolytic processes have drawbacks, for example, that the diamond layer separates from the substrate during electrolysis and that the diamond layer is consumed with the continuation of electrolysis and the amount of the diamond thus consumed is proportional to the current density or quantity of electricity. An improvement in the durability of diamond electrodes has been desired from the standpoint of practical use. The diamond layer consumption is severe especially in aqueous solutions containing organic substances (see Electrochemical and Solid-State Letters, 6(12) D17-D19 (2003)).
Under these circumstances, many investigations have been made also on improvements in the durability of diamond electrodes.
Methods which have been developed for diamond film (layer) synthesis are hot-filament CVD, microwave plasma CVD, the plasma arc jet method, PVD, and the like. In a process generally used for diamond electrode production, polycrystalline diamond is deposited in a thickness of about several micrometers on a substrate such as a valve metal, silicon, carbon material, or the like by CVD.
In the CVD method general employed in diamond electrode production, the substrate undergoes a high-temperature reduction step conducted at about 700-900° C. It is therefore desirable that the thermal expansion coefficient of the substrate be close to that of diamond.
Silicon (Si) is alike to diamond in the thermal expansion coefficient and has the same crystal structure as diamond. Because of this, silicon is being most extensively investigated as the substrate of a diamond electrode. Since relatively large electrodes are necessary in industrial electrolysis, investigations are being made also on the use of metals and carbon materials which are easy to process and have high mechanical strength. In the case of using a diamond electrode, the substrate preferably is stable at noble potentials because the electrode is expected to function as an anode. Valve metals such as titanium and niobium and alloys based on these metals are hence being investigated extensively. Of these, niobium is regarded as a promising material for use as a substrate partly because of its property of being less apt to yield hydrides in a hydrogen atmosphere.
The surface state of a substrate influences the particle diameter and quality of the diamond to be deposited thereon by CVD and interfacial bonding between the diamond layer and the substrate. The surface state is hence thought to be an important factor which exerts a considerable influence on electrode life. In the CVD process, diamond layer deposition occurs from diamond nuclei and the layer grows thereon. Because of this, after the diamond layer formation, the areas where no nuclei were present remain as voids at the interface between the diamond layer and the substrate. It is therefore thought that the presence of diamond nuclei exerts a considerable influence on the life of the electrode.
For imparting the diamond nuclei, use may be made of a method in which a substrate is subjected to a treatment called a marring treatment to impart mars of a nanometer-order size to the substrate surface and diamond nuclei are generated at the mars in the initial stage of a CVD process. Alternatively, use may be made of a method in which a substrate is subjected to the so-called seeding treatment in which fine diamond particles are imparted to the substrate surface prior to a CVD treatment. When a substrate surface is polished with fine diamond particles, marring and seeding are conducted simultaneously.
The surface of a substrate is rugged, and has valleys corresponding to recesses and tops corresponding to protrusions (hereinafter the tops and valleys are inclusively referred to as tops/valleys). With respect to the surface state of the substrate, the following have been found.
(1) Tops/valleys having a depth of 0.002 μm or smaller are eliminated in a CVD process and bring about no effect.
(2) Tops/valleys of 0.002-0.02 μm influence the density of nuclei to be generated.
(3) When a surface shape having tops/valleys of about 0.1-2 μm is formed, this is expected to bring about a longer life than mirror finish.
(4) The effect brought about by tops/valleys of 0.3-1 μm is attributable not to the generation of nuclei but to the so-called anchoring effect, which improves the adhesion strength of the film deposited (see, for example, NEW DIAMOND, Vol.7, 7-13 (1991)).
References 6 to 9 disclose some techniques concerning the surface shape of a substrate for use as an industrial electrode material. However, none of these discloses an effect in diamond electrode production.
Reference 10 discloses a technique of two-layer coating for the purpose of life improvement. Reference 11 discloses a method for adhesion enhancement and substrate protection which comprises forming an interlayer of, e.g., a carbide on the substrate surface.
Also disclosed besides the techniques described in those references is a technique which comprises forming a diamond layer, subsequently removing the substrate, and using the resultant free-standing diamond as an electrode. In this case, diamond layer separation from the substrate, which is one of the problems of diamond electrodes, does not occur. However, it is necessary to form a diamond layer in a thickness sufficient for maintaining a structure as a free-standing electrode, e.g., about several millimeters, resulting in an exceedingly high electrode production cost. Even when the free-standing diamond has a thickness of about several millimeters, the strength thereof is insufficient in electrolytic applications where a relatively large electrode is necessary (see A. Fujishima, Y. Einaga, T. N. Rao, and D. A. Tryk, eds., Diamond Electrochemistry, BKC and Elsevier).
Electrode developments proceed based on the techniques concerning diamond described above. However, no electrode having a satisfactory electrode life in industrial fields, in particular, electrolytic processes, has been commercialized. It has hence become necessary to develop a diamond electrode which has durability for use in industrial electrolysis and can be easily made to have a large size.
[Reference 1] U.S. Pat. No. 5,399,247
[Reference 2] JP-A-7-299467
[Reference 3] JP-A-2000-226682
[Reference 4] JP-A-2000-254650
[Reference 5] JP-A-11-269685
[Reference 6] JP-A-2002-30495
[Reference 7] JP-A-4-301062
[Reference 8] JP-A-3-47999
[Reference 9] JP-A-1-177399
[Reference 10] JP-A-2004-231983
[Reference 11] JP-A-9-268395